The present invention relates to devices for the generation of electrical power and in particular to such devices which incorporate solid oxide fuel cells (SOFC).
A SOFC is an electrochemical device that allows the production of direct current electricity (and/or co-generated heat) by the direct electrochemical combination of a fuel (such as hydrogen, natural gas, coal gas, or other hydrocarbon-based fuels, for example) with an oxidant (such as air). A SOFC consists of an oxygen ion conducting electrolyte (currently based on stabilised zirconia) separating an air electrode (cathode) from a fuel electrode (anode). The fuel is oxidised at the anode and electrons are released to an external circuit, where they are accepted by the cathode. The cathode reaction causes the oxidant gas to be reduced to oxygen ions, which then migrate across the oxygen ion-conducting electrolyte to the anode. The movement of electrons around the external circuit produces an electromotive force (typically 1 volt for a single cell). By the application of a load across the cell, current flows, thus producing a power density, the value of which depends upon the design of the cell and the materials used. The cell is typically run at between 700 and 1000xc2x0 Celcius. The book xe2x80x9cScience and Technology of Ceramic Fuel Cellsxe2x80x9d authors N. Q. Minh and T. Takahashi (Elsevier, Amsterdam, 1995) describes the principle reactions in SOFC, and the methods by which electricity can be produced.
The most attractive features of the SOFC are its high conversion efficiency (typically 50 to 90% if heat utilisation is included), its low production of emissions, its production of high-grade exhaust heat, and its modularisability (from a few kW of electricity to many MW).
Single SOFC cells are usually stacked, using an interconnect or bi-polar plate (usually based on doped lanthanum chromite, or a high temperature metallic system), to produce a multi-cellular unit. The single cell typically produces 1 volt, however by stacking the single cells either in parallel or series connection, the required voltage can be realised. There are several known structures of the SOFC, and these include the planar, tubular and monolithic designs. It should be stressed, however, that in addition to these three main designs, other designs have been sited, although the basic concept remains the same in all designs, that of an oxygen ion conducting electrolyte separating a fuel gas (at the anode) from an oxidising gas (at the cathode). See, for example, the paper xe2x80x9cCeramic Fuel Cellsxe2x80x9d, author N. Q. Minh, pages 563-588, Journal of the American Ceramic Society, 76[3] (1993).
For SOFC systems to become fully realised, and thus fully commercial, they must be reliable over long periods of time, and must not be subjected to thermal cracking due to heating or cooling cycles. The systems must also compete financially with conventional technologies, such as gas turbines and diesel generators, and thus they must be relatively inexpensive and easy to assemble. The major drawbacks of the current designs are principally based on sealing of the single cells. The planar design, using flat plate electrolytes with an anode and cathode adhered to either side, appears to be the cheapest system to fabricate, however its major problem still hinges around the issue of sealing the plates without causing excessive stressing of the ceramic plates, or chemical compatibility issues between the sealant and the cells. The tubular design gets around the problem of sealing by using a closed or open-ended tube. In the conventional tubular design, an extruded porous doped-lanthanum manganite support tube is fabricated. The electrolyte (stabilised zirconia) is electrochemically vapour deposited onto the support tube. The anode is then slurry spray-electrochemical vapour deposited onto the electrolyte, and doped-lanthanum chromite is plasma sprayed onto the cell as an interconnect material. The cells are then bundled into multi-cellular units and then packaged as a SOFC system. The air is pumped into the interior of the tubes, while the exterior of the tube is exposed to the fuel gas. The tubes are sealed at one end, so that spent air can flow back through an annular. The spent fuel can also be recycled to allow for heat recovery.
This tubular arrangement has been very successful, However, the design does not allow for rapid heat cycling, as thermal stresses could occur causing the cells to crack. Even with the improvements that have been undertaken, the current limit on this design requires 5 hours for the system to reach 1000xc2x0 C. operating temperature from ambient. State-of-the-art generators, using the technology described above, are described in U.S. Pat. No. 5,244,752. These are significant improvements on those SOFC systems sited in U.S. Pat. Nos. 4,374,184, 4,395,468, 4,664,986, 4,729,931, and 4,751,152. The design described above is also expensive to fabricate and is not conducive to producing small-scale systems (sub-10kW). The manufacturers of the systems have reduced the manufacturing costs by reducing the cost of the raw materials. For example, 90% of the weight of the cell is in the doped-lanthanum chromite air electrode, and thus by using a cheaper supply of raw materials (high impurity level), the cost can be reduced substantially. As described in a paper xe2x80x9cRecent Progress in Tubular Solid Oxide Fuel Cell Technologyxe2x80x9d, author S. C. Singhal, in Solid Oxide Fuel. Cells Volume V, The Electrochemical Society (New Jersey), pages 37-50 (1997). However, the cost of the system will not allow for small-scale production.
To overcome the problem of high manufacturing cost, the use of extruded thin walled stabilised zirconia tubes is disclosed. See for example Australian Patent 675122. The inner electrode in this design was the fuel electrode, while the outer electrode was the air electrode (usually lanthanum manganite). In the design, tubes were supported within a thermally insulating container from which the exhaust gas can escape through a passageway. In this design, the cost of the tubes has been reduced by using a simple extrusion technique for extruding stabilised zirconia mixed with, for example, polyvinyl butyral and cyclohexane. The design is described as containing an array of the aforementioned tubes, supported within a thermally insulating container from which the combustion products can escape through a passageway. Gas is supplied directly to the top of the tubes. The combustion products escape through the same passageway as the forced air inlet. Although very simple, this design does not allow for the full movement of the cells within the reactor, and it is therefore still liable to stress build up in the cells, which may cause cell damage.
The ability to reform the fuel within the SOFC generator is also not possible in the designs described above. Reforming is a process by which the fuel, in this case usually a hydrocarbon fuel, is combined with water and/or carbon dioxide to produce carbon monoxide and hydrogen. This reformed fuel is then directly used in the SOFC system. In the majority of cases, the fuel is reformed outside the SOFC generator, which requires expensive equipment such as heat exchanges, pumps etc., which also make the whole system a lot more bulky. The reforming reaction, when it takes place outside the generator is highly undesirable as a lot of energy is lost from the system (as heat) and thus causes an overall loss in the efficiency of the system, and an increase in its complexity. This was partially overcome in the U.S. Pat. No. 4,729,9331, where the reforming of a reformable gaseous fuel was performed in the SOFC generator. In this system, the partially spent fuel is divided into two streams; one is combined with the partially spent air stream to form exhaust gas, which is then partially vented. Some of the remaining exhaust gas is then combined with the second spent fuel stream. The combined stream is then mixed with the gaseous reformable fuel. The invention uses the heat balance in the system as a whole to minimise the heat losses. The invention uses the traditional design of the tubular SOFC systems, as described in the U.S. Patents described above, and does not, therefore, overcome the original problems of complexity of system, expense, and the ability to thermally cycle at fast heating and cooling rates. The system is also quite complicated.
An alternative design is described in U.S. Pat. No. 3,377,703. Here, several electrolyte tubes stand upright on a ceramic base. Gas is passed through the tubes and combusted at the top of the tubes. The hot gases then flow around the system to a heat exchanger. Although relatively simple, the system requires a significant number of high temperature seals, and will not allow for high thermal stresses, as may be experienced in rapid start-up.
It is therefore an object of the present invention to provide a power generating apparatus incorporating solid oxide fuel cells which goes some way toward overcoming the above disadvantages or will at least provide the public with a useful choice.
In a first aspect the invention consists in apparatus for generating electrical power comprising:
a plurality of tubular solid oxide fuel cells in a reaction chamber, each said cell having an inwardly facing reaction surface and an outwardly facing reaction surface, and electrodes associated with each said surface, and having one end of the said cell mounted in a manifold block,
a first gases inlet path to supply first gases to said reaction chamber, said fist gases to pass along the outside of said fuel cells,
a second gases inlet path to supply a second gases to the mounted ends of said fuel cells to pass along the inside of said fuel cells,
an exhaust gases outlet path from said reaction chamber adjacent the ends of said fuel cells away from said manifold block, for carrying exhaust gases to an exhaust outlet and in which unreacted gases may combust with one another, and
power collection means which connect to said conductors on the insides and outsides of said fuel cells;
one of said first and second gases being reformable fuel gases and the other being oxidising gases, said gases paths thereby each for carrying one of reformable gases and oxidising gases in accordance with the arrangement of the reactive surfaces of said fuel cells,
said reformable gases carrying path including a reformation catalyst therein and being arranged in heat transfer relation with said exhaust gases path and said reaction chamber such that in steady state operation the gases therein may be raised to a temperature at said catalyst at which reformation can occur, and
said oxidising gases carrying path is arranged in beat transfer relation with said exhaust gases path and said reaction chamber such that in steady state operation the gases therein may be raised to a reaction temperature at said fuel cells at which said fuel cells operate.
Each said tubular fuel cell may have an outside diameter between 3 mm and 10 mm and has a wall thickness of between 0.3 mm and 1 mm.
A variable flow restriction means may be provided in said oxidising gas inlet path to variably restrict the flow of said oxidising gas to said fuel cells and said combustion zone, to thereby control the combustion temperature.
The second gas inlet path may include in part a cylindrical or conical chamber, and said exhaust gases outlet path includes in part a cylindrical or conical chamber concentric with said inlet path and divided therefrom by a wall of a material typically alumina or mullite, or with a thermal impedance less than the thermal impedance of such materials.
The first gas inlet path may include at least in part a cylindrical channel divided from said oxidising gas inlet path by a wall of a material typically alumina or mullite, or with a thermal impedance less than the thermal impedance of such materials.
A baffle wall made divide said reaction chamber from said exhaust gases outlet path, said tubular cells extending through apertures in said baffle wall into said exhaust gases outlet path, and apertures in said baffle wall allowing a flow of partially reactive gases to flow from said reaction chamber to said exhaust gases outlet path to combust with the partially reacted gases flying from the insides of said tubular cells.
Each said tubular fuel cell may be held only at the end thereof mounted in said manifold block.
Each said cell may pass through an aperture in said baffle wall, being spaced clear from the edge of said aperture all around said cell, and said partially reacted gases which flow from said reaction chamber to said exhaust gases outlet path may flow through said apertures, through the space between the wall thereof and the respective said fuel cell.
The first gases inlet path may include a plenum chamber below said manifold block, and said fuel cell mounted ends may extend through said manifold block, said plenum chamber dispersing said gases to said fuel cell mounted ends.
The first gases may be said oxidising gases and said power connection means may connect to said electrodes on said insides and outsides of said fuel cells at the ends thereof disposed within said plenum chamber.
The first gases inlet path may include a thermally conductive gases conduit extending into said plenum chamber through said manifold block, a first length of the conduit passing through said reaction chamber to absorb heat from the gases in said reaction chamber under steady state operating conditions.
A second length of said thermally conductive gases conduit may pass through at least part of said exhaust gases outlet path.
An annular chamber may surround said reaction chamber, with gases inlet port means through which gases flow through said annular chamber and gases outlet port means for gases to flow from said annular chamber into said reaction chamber at the far end of said annular chamber from said inlet port means, said gases outlet port means located adjacent said manifold block, and the wall between said reaction chamber and said annular chamber being thermally conductive, for example being formed from alumina or mullite or having a similar thermal impedance to those materials.
One or more mixing ports may be provided in said wall between the annular chamber and the reaction chamber, the ports leading between the chambers at the far end of the reaction chamber from the gases outlet port means, and a suction generation means be provided to create a low pressure zone in the annular chamber immediately adjacent the mixing ports under the action of gases which pass through the annular chamber to draw gases through the mixing ports from the reaction chamber into the annular chamber. Such suction generation means may comprise some form of venturi.
The fuel cells may be based on a yttria stabilised zirconia electrolyte, mounted in a glass ceramic insulator (such as that sold under the MACOR brand by Corning Glassworks), and the manifold block be manufactured from ferritic stainless steel.
An ignition means may be provided within the combustion zone to generate a localised heat sufficient to ignite said fuel gas in the presence of said oxidising gas.
Control means may control said ignition means and the flow of said fuel gases to cause said fuel gases to flow at a substantially higher rate during start up than that required for power generation, ignite said gas flow and maintain said gas flow at said substantially high rate until a temperature of approximately 800xc2x0 C. is reached at said manifold block.
For commonly available SOFC compositions the reaction temperature would be between 400xc2x0 C. and 600xc2x0 C. For a standard reforming reaction with commonly used nickel catalyst the reforming temperature would be between 600xc2x0 C. and 800xc2x0 C.
In a further aspect the invention consists in a space heater including power generating apparatus as described above, and wherein said exhaust gases are utilised for space heating and said generated power is distributed for other applications.
In a yet further aspect the invention consists in a water heating cylinder incorporating a power generation apparatus as described above, and wherein said exhaust gases are used for heating a supply of water in said water heating cylinder and said generated power is distributed for other applications.
In a still further aspect the invention consists in power generating apparatus substantially as herein described with reference to and as illustrated by the accompanying drawings.
To those skilled in the art to which the invention relates, many changes in construction and widely differing embodiments and applications of the invention will suggest themselves without departing from the scope of the invention as defined in the appended claims. The disclosures and the descriptions herein are purely illustrative and are not intended to be in any sense limiting.